Final answer:
In a healthy pregnancy, progesterone levels are variable; initially high to prevent contractions and then decrease in the later stages, contributing to the onset of labor.
Step-by-step explanation:
The progesterone level for a healthy pregnancy can be best described as variable (Option C). Progesterone is essential for maintaining pregnancy as it inhibits uterine contractions during the first several months. However, as the pregnancy progresses into the seventh month, progesterone levels plateau and then decrease, making the myometrium more sensitive to contraction-inducing stimuli. Estrogen levels rise in the maternal circulation, and this increasing estrogen-to-progesterone ratio, along with a rise in fetal cortisol during the eighth month, further diminishes progesterone's calming effects on the uterus.
Before pregnancy, progesterone is produced by the ovaries. During the third trimester, the placenta takes over the production of progesterone, which, along with human beta chorionic gonadotropin (β-HCG), helps maintain a successful pregnancy.